1. Field of the Invention
The present invention relates to a process for the preparation of aromatic aminoazo compounds by diazotizing an aromatic amine with vaporous oxides of nitrogen, by coupling the diazonium salt with an aromatic amine and by rearranging the aromatic diazoamino compound which is formed as an intermediate. The process according to the invention is especially well adapted to the preparation of p-aminoazobenzene from aniline.
2. Description of the Prior Art
Processes for the preparation of aromatic aminoazo compounds from an aromatic amine are described in the literature. These processes have been carried out in a single or plural stage. Thus, according to Fierz-David [Grundlegende Operationen der Farbenchemie (Principal Operations in Dyestuff Chemistry), 8th edition, 1952, pages 265-6], p-aminoazobenzene is obtained in three stages from aniline, by first diazotizing aniline at a low temperature, then by coupling the diazonium salt with aniline and by separating diazoaminobenzene [or triazene], in the solid state, after precipitation; in a final stage, the triazene is rearranged into p-aminobenzene by dissolving the triazene in aniline and adding aniline hydrochloride thereto.
Attempts have subsequently been made to obtain the aromatic aminoazo compounds by reducing the number of stages and by carrying out the process under conditions in which the diazonium salt and/or the triazene were converted directly in the medium as they were formed. Thus, in East German Pat. No. 77/985 it has been recommended to carry out all three stages simultaneously by injecting hydrochloric acid into a mixture of aniline, water and sodium nitrite, which has been heated to a temperature about 50.degree. C. and generally of about 100.degree. C. This process is carried out at a high temperature in the presence of relatively large amounts of water. A certain number of disadvantages result, one of which, in particular, is a lowering of the yield of p-aminoazobenzene through by-production of a relatively large amount of o-aminoazobenzene.
Aminoazobenzene hydrochloride has also been successfully obtained in a single stage from aniline, by use of an alcohol, having at most three carbon atoms, under very specific conditions of concentration. This technique is described in U.S. Pat. No. 2,538,431. However, the presence of an alcohol favors the secondary reactions and increases the complexity of the process. The reaction must also be permanently monitored in order to prevent the formation and accumulation, in the medium, of an alkyl nitrite, the latter being well known for its instability.
French Pat. No. 1,192,374 to Goodrich describes a two-stage process for the preparation of an aromatic aminoazo compound, and essentially of p-aminoazobenzene, by adding nitrite, at between 0.degree. and 50.degree. C., to a reaction medium comprising aniline, aniline hydrochloride and water in proportions such that there are from 2.5 to 12 mols of aniline and 1.03 to 2.5 molecular equivalents of acid per mol of nitrite employed. The diazoaminobenzene is rearranged into aminoazobenzene in a second stage carried out at between 40.degree. and 75.degree. C.; the rearrangement reaction is preferably carried out in the presence of a Lewis acid. The Lewis acid accelerates the rearrangement process. This process, which requires that two stages be carried out at different temperatures, makes it possible to obtain p-aminoazobenzene with a yield of the order of 90%, the reaction generally taking one or several hours. In this process, the use of aluminium chloride leads to significant complications from a technical point of view, and the conversion ratio of the triazene is not quantitative; aminoazobenzene containing a small amount of triazene is therefore obtained upon completion of the reaction, which constitutes, on the one hand, a loss in yields, and, on the other hand, a considerable disadvantage as regards the purification of the p-aminoazobenzene.
Other techniques have proposed to first prepare the diazonium salt or the triazene and to convert the latter into an aromatic aminoazo compound in a single stage. Thus, K. H. Meyer [Berichte, 54, 2,265-2,272 (1921)]has described the possibility of obtaining p-aminoazobenzene from a benzenediazonium halide and aniline hydrochloride by adding acetic acid and a concentrated aqueous solution of sodium formate; however, the yields are low.
It has also been shown that it is possible to obtain p-aminoazobenzene directly from benzenediazonium chloride and aniline, when aniline hydrochloride and the diazonium chloride are reacted in an aqueous medium containing free hydrochloric acid. However, this is an excessively slow reaction which is carried out in a homogeneous medium and gives a very poor yield [H. V. Kidd, Journal of Organic Chemistry, 2, 192-207 (1937)].
Finally, Goldsmith [Zeitschrift fur physikalische Chemie, 110, 251-265 (1924)] has proposed to convert triazene into p-aminoazobenzene in a medium essentially comprising aniline and containing small amounts of aniline hydrochloride and, optionally, small amounts of water. It was shown that the presence of water decreased the rate of the rearrangement reaction.
From the chemical literature, it is thus found that it is the rearrangement reaction which appears to be the slow and incomplete reaction of all the diazotization, coupling and rearrangement reactions. However, attempts have been made to activate this rearrangement reaction, either by adding catalysts [technique of French Pat. No. 1,192,374], or by carrying out the reaction in the presence of very well defined amounts of an aliphatic alcohol having at most three carbon atoms [process described in U.S. Pat. No. 2,538,431]. However, these techniques have not made it possible to carry out a rapid and total conversion of the aromatic diazoamino compound into the aromatic aminoazo compound with quantitative yields.
French Pat. No. 2,306,977 to American Cyanamid describes a process for the preparation of an aromatic aminoazo compound by mixing in a tubular reactor a first stream of reagents containing a great excess of aromatic amine, a mineral acid in stoichiometric excess, and water, with a second stream consisting of an aqueous solution of alkali nitrite. Thus, in good yields, and with kinetic rapidity, the aromatic aminoazo compound is prepared. Nonetheless, certain inorganic compounds, e.g., salts such as sodium chloride, are also obtained in stoichiometric proportion. The presence of such, e.g., salts, typically effects heterogeneity in the medium which reduces the conversion of the reaction intermediates. It too is necessary, in order to limit pollution, to separate and purify inorganic compounds.